It is well known that semiconductor devices fabricated from SiC are in principle able to operate at very high temperatures, since SiC has a high thermal stability, so that it will have a stable function at much higher temperatures than Si, namely well up to 1000.degree. K. In addition, for semiconductor device made of SiC it is necessary to anneal the layer next to a contact at comparatively high temperatures, namely at least 800.degree. C. when this layer has a very high doping concentration and otherwise even higher, to form in this way a high density of interface defects next to the contact layer and by that an ohmic contact. The need of such a high temperature anneal requires the use of a refractory metal in devices where a metal layer has to be placed on the SiC layer with an insulating layer therebetween, such as a gate contact in gate controlled devices as MOSFETs and IGBTs or field rings for reducing the electric field at the edges of a semiconductor device. "Refractory metal" is here defined as a metal having a low reactivity at high temperatures.
It has been found that refractory metal nitrides, such as MoN, TaN and particularly TiN would be well suited for forming such metal layers. They are especially suitable for forming the gate contact of a gate controlled semiconductor device of SiC, and this particular application will hereinafter be discussed in order to illuminate but in no way restrict the invention and the problems to be solved thereby.
As already mentioned, it has previously been recognized that a possible gate contact could be formed using deposited TiN, and this has been disclosed in for instance, EP 0 697 714. Besides the fact that TiN is a refractory metal, it is also an efficient diffusion barrier which should limit extrinsic degradation. However, experiments have shown that although a stable gate contact may be formed using TiN on SiO.sub.2, this appears to be only stable up to 600.degree. C. For the reason mentioned above, it is not stable for the high temperatures necessary for ohmic contact anneals and unfortunately neither sufficiently stable for long term high temperature operation of such a semiconductor device. This is due to the fact that Ti is very reactive with respect to oxygen, and it has been identified that there will be an interface reaction of SiO.sub.2 with TiN to form TiO.sub.2, which is a contributing instability mechanism. This problem may also be there for the other refractory metal nitrides that may be considered.